Zeolite catalysts have been used and proposed for use in a number of different petroleum refining processes such as cracking, for example, as described in U.S. Pat. Nos. 3,700,585 and 3,907,663, hydrocracking as described in U.S. Pat. No. 3,923,641, dewaxing and hydrodewaxing as described in U.S. Pats. Nos. Re. 28,398, 3,700,585, 3,956,102, 4,110,056 and 3,755,138, aromatization processes of the kind described in U.S. Pats. Nos. 3,806,443, 3,767,568, 3,753,891, 3,770,614 and 3,843,740 and alkylation as described in U.S. Pat. No. 3,641,177. They have also found utility in the petrochemical industry, for example, in alkylation processes of the kind described in U.S. Pat. Nos. 3,668,264, 3,251,897, 4,117,024, 4,049,738 and 4,086,287, isomerization processes of the kind described in U.S. Pats. Nos. 4,100,214 and 4,101,596 and disproportionation processes as described, for example, in 4,106,788 and 3,856,871. Their use in the production of hydrocarbons from other materials such as synthesis gas, methanol, dimethyl ether (DME) or other oxygenated materials is described, for example, in U.S. Pat. Nos. 3,894,102 to 3,894,107, 3,899,544, 4,039,600, 4,048,250 and 4,035,430. In these processes various kinds of zeolites may be used either alone or in combination with one another or with other catalytic materials. Zeolites may be characterized as being small pore materials such as erionite or zeolite A; large pore materials such as zeolite X, zeolite Y or mordenite and the so-called shape selective zeolites exemplified by the ZSM-5 family including ZSM-5 itself, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38.
One problem which has persisted with zeolitic catalysts is that of hydrothermal stability. In many of the processes in which they are used, the zeolites are exposed to water vapor at elevated temperatures and this may tend to reduce the activity of the zeolite by reason of the loss of acidic sites through dehydroxylation and dealuminization, the loss being manifested by a decrease of the alpha value of the zeolite. In some cases, particularly with zeolites of low silica:alumina ratio, crystallinity may be adversely affected. Different zeolites exhibit different degrees of hydrothermal stability but the problem is encountered to some extent with all of them. The exposure to the water vapor may occur during the process itself or in an ancillary treatment step. For example, in the hydrocarbon synthesis processes of the kind described in U.S. Pat. Nos. 3,894,102 to 3,894,107, 3,899,544 and 4,035,430 using oxygenated precursors such as methanol or DME, large amounts of water are produced as by-products of the synthesis and under the reaction conditions commonly employed this will be evolved as steam which will come into direct contact with the catalyst, to its ultimate detriment. In other processes such as catalytic cracking, the steam stripping which is used to remove occluded hydrocarbons prior to regeneration, will obviously produce a similar effect, as will any steam which is present in the regeneration and which has been produced either by combustion of any hydrocarbon material on the catalyst itself or by the combustion of hydrocarbon fuel used to heat the regenerator. Obviously, the deleterious effect of the steam becomes more pronounced the longer and more frequent the exposure to it is; processes in which the catalyst is continuously or continually exposed to steam therefore present greater problems than those where the contact is occasional or at very long intervals. For example, in fluid catalytic cracking (FCC) units the catalyst is continuously circulated through the reactor and the regenerator and comes into contact with steam during each complete cycle when the catalyst is subjected to the stripping and regeneration steps. Conversely, processes in which the exposure to steam is infrequent, e.g. the alkylation process of U.S. Pat. No. 4,276,438 where regeneration is carried out about once a year, usually present few problems of hydrothermal stability, at least when water is not one of the by-products of the reaction.
Attempts to improve the hydrothermal stability of zeolites have often been made and have met with varying success; although it has often been found possible to improve the hydrothermal stability, other properties of the zeolite may be adversely affected. For example, various cationic forms of the zeolite such as the rare earth form of the faujasites zeolites have greater hydrothermal stability but the activity of the zeolite in the rare earth form may not be as great as it would be in other forms, and it may not always be practicable or possible to improve the hydrothermal stability in this way. For example, the rare earth cations do not readily enter the structure of the ZSM-5 type zeolites because of the low ion exchange selectivity of these cations. It would therefore be desirable to find a way of improving the hydrothermal stability of these and other zeolites.